Related subject matter is disclosed in the following application filed concurrently herewith and assigned to the same assignee hereof: U.S. patent application entitled xe2x80x9cObtaining Round Trip Time Delay Parameter For A Wireless Terminal Of An Integrated Wireless-Global Positioning Systemxe2x80x9d, Ser. No. 09/552,897.
The present invention relates generally to an integrated wireless-global positioning system and, more particularly, to obtaining and selectively using the pilot phase offset time delay parameter for a wireless terminal of an integrated wireless-global positioning system.
Global positioning systems, which are satellite based, provide accurate, three dimension position information to worldwide users. FIG. 1 depicts a global positioning system (GPS) 10. The GPS 10 comprises a plurality of satellites 12-j and at least one GPS receiver 14, where j=1,2, . . . , n. Each satellite 12-j orbits the earth at a known speed vj and is separated by a known distance from the other satellites 12-j. Each satellite 12-j transmits a global position signal 11-j which includes a carrier signal with a known frequency f modulated with a unique pseudo-random noise (PN-j) code and navigational data (ND-j) associated with the particular satellite 12-j. The PN-j code includes a unique sequence of PN chips and navigation data ND-j which includes a satellite identifier, timing information and orbital data, such as elevation angle xcex1j and azimuth angle xcfx86j. FIG. 2 depicts a typical 20 ms frame of the GPS signal 11-j which comprises twenty full sequences of a PN-j code in addition to a sequence of navigation data ND-j.
GPS receiver 14 comprises an antenna 15 for receiving GPS signals 11-j, a plurality of correlators 16-k for detecting GPS signals 11-j and a processor 17 having software for determining a position using the navigation data ND-j, where k=1,2, . . . , m. GPS receiver 14 detects GPS signals 11-j via PN-j codes. Detecting GPS signals 11-j involves a correlation process wherein correlators 16-k are used to search for PN-j codes in a carrier frequency dimension and a code phase dimension. Such correlation process is implemented as a real-time multiplication of phase shifted replicated PN-j codes modulated onto a replicated carrier signal with the received GPS signals 11-j, followed by an integration and dump process.
In the carrier frequency dimension, GPS receiver 14 replicates carrier signals to match the frequencies of the GPS signals 11-j as they arrive at GPS receiver 14. However, due to the Doppler effect, the frequency f at which GPS signals 11-j are transmitted changes an unknown amount xcex94fj before the signal 11-j arrives at the GPS receiver 14. Thus, each GPS signal 11-j will have a frequency f+xcex94fj when it arrives at the GPS receiver 14. To compensate for the Doppler effect, GPS receiver 14 replicates the carrier signals across a frequency spectrum fspec ranging from f+xcex94fmin to f+xcex94fmax until the frequency of the replicated carrier signal matches the frequency of the received GPS signal 11-j, wherein xcex94fmin and xcex94fmax are a minimum and maximum change in the frequency the GPS signals 11-j will undergo due to the Doppler effect as they travel from satellites 12-j to GPS receiver 14, i.e., xcex94fminxe2x89xa6xcex94fjxe2x89xa6xcex94fmax.
In the code phase dimension, GPS receiver 14 replicates the unique PN-j codes associated with each satellite 12-j. The phases of the replicated PN-j codes are shifted across code phase spectrums Rj(spec) until the replicated carrier signals modulated with the replicated PN-j codes correlate, if at all, with the GPS signals 11-j being received by the GPS receiver 14, where each code phase spectrum Rj(spec) includes every possible phase shift for the associated PN-j code. When the GPS signals 11-j are detected by the correlators 16-k, GPS receiver 14 extracts the navigation data ND-j from the detected GPS signals 11-j and uses the navigation data ND-j to determine a location for the GPS receiver 14.
A GPS enables a ground based receiver to determine its position by measuring the time difference required for GPS signals initiated from two or more satellites to be received by a wireless terminal. The pseudorange is defined as this time difference times the speed of light. The pseudorange is not the real range because it contains errors caused by the receiver clock offset. To determine a two-dimensional position (latitude and longitude) usually requires receiving signals from three satellites. To determine a three-dimensional position (latitude, longitude, and altitude) requires receiving pseudoranges from four or more satellites. This precondition, however, may not always be satisfied, especially when the direct satellite signals are obstructed, such as when a wireless terminal is inside a building.
GPS receivers are now being incorporated into wireless telephones or other types of mobile communication devices which do not always have a clear view of the sky. In this situation, the signal-to-noise ratios of GPS signals 11-j received by GPS receiver 14 are typically much lower than when GPS receiver 14 does have a clear view of the sky, thus making it more difficult for GPS receiver 14 to detect the GPS signals 11-j.
Integrated wireless-global positioning (WGP) systems were developed to facilitate the detection of GPS signals 11-j by GPS receivers. The WGP system facilitates detection of GPS signals 11-j by reducing the number of integrations to be performed by correlators searching for GPS signals 11-j. The number of integrations is reduced by narrowing the frequency range and code phase ranges to be searched. Specifically, the WGP system limits the search for GPS signals 11-j to a specific frequency or frequencies and to a range of code phases less than the code phase spectrum Rj(spec).
The position of a wireless terminal may also be determined from information obtained from a wireless network. The information typically includes pilot phase offset (PPO) signals. The PPO measurement contains the information of the distance between the wireless terminal and the BS. Pilot phase offset is the measurement of the code phase in a pilot signal. It consists of information of the distance between the wireless terminal and the BS plus a bias that is the same for all pilot phase offset (PPO) measurements from any one base station. If pilot phase offset measurements from two BSs are available, two pilot phase offset measurements can be used to construct one pilot phase offset measurement by subtracting one pilot phase offset measurement from the other. The subtraction cancels out the unknown constant, and so the PPO measurement is the distance from the wireless terminal to one BS minus the distance from the wireless terminal to the other BS. If three of more PPO measurements are available, the 2D position of a wireless terminal may also be determined from triangulation schemes. One problem with using wireless network based signals to determine the location of a wireless terminal is that the measurement errors of the PPO is usually much larger than the satellite based navigational system measurement errors. Another problem is that three or more measurements may not always be available for the purpose of position determination.
An integrated wireless-global positioning (WGP) system relies on both the satellite navigation system and the wireless communication system to determine the location of a wireless terminal. The integrated wireless-global positioning system combines the data from both the wireless network and the satellites navigation system to obtain an integrated position solution. By combining information from both the global positioning system and the wireless network, it is possible to increase the positioning accuracy and, at the same time, overcome the requirement of having at least three measurements.
FIG. 3 depicts an integrated wireless global positioning system 20 comprising a WGP server 22, a plurality of base stations 23 and at least one WGP client 24. WGP server 22 includes a GPS receiver 26 having an antenna 27 installed in a known stationary location with a clear view of the sky. WGP server 22 is operable to communicate with base stations 23 either via a wired or wireless interface. Each base station 23 is at a known location and provides communication services to WGP clients located within a geographical area or cell 25 associated with the base station 23, wherein each cell 25 is a known size and may be divided into a plurality of sectors. WGP client 24 includes a GPS receiver 28 and perhaps a wireless terminal such as a wireless telephone 29, and is typically in motion and/or at an unknown location with or without a clear view of the sky.
FIG. 4 is a flowchart 300 illustrating the operation of WGP system 20. In step 310, WGP server 22 detects a plurality of satellites 12-j via their GPS signals 11-j using its GPS receiver 26. WGP server 22 acquires the following information from each detected satellite 12-j: the identity of satellite 12-j and frequency fj, code phase, elevation angle xcex1j and azimuth angle xcfx86j associated with the detected satellite 12-j, wherein the elevation angle xcex1j is defined as the angle between the line of sight from WGP server 22 or client 24 to a satellite 12-j and a projection of the line of sight on the horizontal plane, and the azimuth angle xcfx86j is defined as the angle between the projection of the line of sight on the horizontal plane and a projection of the north direction on the horizontal plane. See FIG. 5, which depicts an elevation angle xcex1j and an azimuth angle xcfx86j corresponding to a satellite 12-j and a WGP server 22 or WGP client 24.
In step 315, WGP server 22 receives sector information from base station 23 currently in communication with or serving WGP client 24, wherein the sector information indicates the sector WGP client 24 is currently located. In step 320, WGP server 22 makes an initial estimate of WGP client""s position based on the known location of the serving base station, the cell size associated with the serving base station, and the sector in which WGP client 24 is currently located. In one embodiment, WGP server 22 initially estimates that WGP client 24 is located at a reference point within the sector, e.g., point at approximate center of sector. In another embodiment, WGP server 22 initially estimates WGP client 24""s position using known forward link triangulation techniques.
In step 330, for each detected satellite 12-j, WGP server 22 uses the information acquired from the detected GPS signals 11-j to predict a frequency fj(r) at the reference point and a code phase search range Rj(sect) which includes all possible code phases for GPS signal 11-j arriving anywhere within the sector where WGP client 24 is currently located. In step 340, WGP server 22 transmits a search message to the serving base station 23, where the search message includes, for each detected satellite 12-j, information regarding the associated PN-j code, predicted frequency fj(r) and code phase search range Rj(sect).
In step 350, serving base station 23 transmits the search message to WGP client 24 which, in step 360, begins a parallel search for the satellites 12-j indicated in the search message. Specifically, WGP client 24 will use its correlators to simultaneously search for each of the GPS signals 11-j at the predicted frequency fj(r) within the limitations of the code phase search range Rj(sect) indicated in the search message.
The performance of an integrated wireless-global positioning system depends directly on the quality and accuracy of the information received from the satellite and the wireless network, namely, the pseudoranges, the PPO, etc. Unfortunately, these measurements are normally noisy and frequently contain relatively large bias errors. This is particularly true for wireless network measurement signals which frequently have measurement errors, a large portion of which are biases. The biases are defined as time delays which occur during the transmission and processing of signals and will normally vary for each sector, for each base station, and for each type and model of wireless terminal being used. Accurately estimating and calibrating these time delays is important for the successful operation of an integrated wireless-global positioning system.
For small scale network systems in which only several base stations/wireless terminals are involved, a system can be setup to calibrate the time delays for each base station and/or wireless terminal for a GPS. One approach is to measure the individual time delays of each wireless terminal, the base station, and the multipath separately. This approach requires a large team of technically trained people equipped with precise and expensive instruments to measure the delays for each type of wireless terminal in each sector of each base station. This approach is costly, time consuming, and if not done with great care, could result in poor performance. However, the process of actually calibrating the time delays for each base station is not feasible for large scale commercial network systems where hundreds of base stations and dozens of different types of wireless terminals are involved.
Another approach is to estimate the network delay parameters based on the wireless network and satellite measurements. A system is established to estimate and adjust the time delay parameters, and then store this information in a database. Once the database of the time delay parameters is established, it is used to assist in determining the location of wireless terminals whose positions are not known. The estimation of the time delay parameters for a wireless terminal/cell sector pair involves typically the following steps:
(1) Setup the wireless terminal to be calibrated at a location where it has an unobstructed reception of satellite signals and wireless network signals in the main antenna beam direction of the cell sector to be calibrated. The location should not be too far from the base station to avoid the multipath influence; and should not be too close to the base station to avoid the problem that the wireless terminal might be locked onto the wrong signal peak instead of the main signal peak.
(2) Survey the location to get the precise coordinates of the wireless terminal. If the coordinates of the base station antenna are unknown, they should be determined. The coordinates of the wireless terminal should be accurate to several meters or better. The positioning accuracy of a commercial differential GPS can typically be one meter or better.
(3) Collect a large number of network and satellite measurements. To have a unobstructed statistical estimate of the time delay parameters, more than one hundred samples should be collected.
(4) Calculate the time delay parameters based on the known positions of the wireless terminals and the base station, and the collection network and satellite measurements.
(5) Adjust or store the time delay parameters in the database in terms of new estimates. The database stores the time delay information for each type of wireless terminal, and also for each cell sector of every base station.
Clearly, the above noted prior art procedure is both costly and time consuming. It requires going through the same procedures to calibrate each type of wireless terminal of each sector for every base station. Because of the time constraints and cost involved for each calibration, the number of the samples that can be obtained is quite limited which, in turn, effects the accuracy of the result obtained. Furthermore, with this calibration process, hardware/software changes which occur within the network and the wireless terminal could require that the calibration process be repeated. Therefore, the above noted calibration process is suitable only for small-scale wireless network systems having only several base station/wireless terminals. It is not practical for large scale commercial network systems where hundreds of base stations and dozens of different types of wireless terminals must be calibrated.
Accordingly, a need exists for a method of automatically obtaining the PPO time delay parameters for wireless terminals in an integrated wireless-global positioning system and, thereafter, selectively using the PPO time delay parameter to determine the position of a wireless terminal which does not have unobstructed reception of the satellite signals.
An integrated wireless-global positioning (WGP) system determines the location of a wireless terminal from satellite measurements and wireless network measurements. The accuracy of the location identified is dependent upon the quality of the measurement from the wireless network which, unfortunately, can contain measurement errors such as biases. Biases are caused by time delays which occur during the processing and transmission of signals. The time delays vary for each sector/base station and for each type of wireless terminal model being used. Calibration of the biases is important for the success of the WGP system. In this invention there is disclosed a method for using the WGP system to automatically determine the pilot phase offset time delay parameters (biases) and to adaptively adjust the parameters for changes that may occur. The method does not require that a particular system be set up for the calibration, and it does not require a team of technical experts to perform field calibrations. With this method, the wireless terminals that have unobstructed reception of GPS signals are used to obtain pilot phase offset time delay parameters and to then use the time delay parameter to calibrate the pilot phase offset measurements when determining the position of a wireless terminal that does not have unobstructed GPS signal reception.